1. Field of the Invention
The present invention relates to a fixing device that fixes a toner image onto a recording sheet by head and pressure.
2. Description of the Related Art
In an image forming apparatus that uses the electrophotography method, electrostatic latent images are formed on the surface of a photosensitive drum functioning as an image carrying member, the electrostatic latent image is converted to a visible toner image by a developer, the toner image is transferred to a recording sheet by a transfer device, and borne to a fixing device, which fixes the toner image onto the recording sheet by applying heat and pressure. The recording sheet bearing the fixed image is then transported over a discharge path and discharged out of the image forming apparatus.
The fixing device includes a pair of opposing fixing members. The fixing members can be rotating members such as rollers or belts, or stationary members such as pressure pads, or a combination thereof. The recording sheet is clamped between the opposing fixing members and the toner image is fixed onto the recording sheet by applying heat and pressure to both the recording sheet and the toner image.
Of the fixing members, one is typically a heating member, i.e., a fixing roller, with a built-in heater and the other is a pressure lever in the form of a pressure exerting roller that presses against the fixing roller. Upon arriving at the fixing device, the recording sheet bearing the toner image thereon is clamped in a fixing nip formed by the contact portions of the fixing roller and the pressure exerting roller. As the recording sheet passes through the fixing nip, the toner image is fixed to the recording sheet due to heat and pressure from the fixing roller and the pressure exerting roller.
However, when there is jamming of the transfer sheet, if the apparatus stops with the recording sheet stuck in the fixing nip of the fixing device, the jam release operation to release the sheet from the fixing nip can be a tough task. To ease the jam release operation, a well known method of using a manual release lever to manually release the jam is widely used.
However, the manual release lever can be cumbersome to operate and there is a likelihood that the user may forget to operate the lever. To take care of these issues, Japanese Patent Application Laid-open No. H5-173446 discloses an automatic jam releasing device in the form of a cam device that automatically loosens the nip pressure at the fixing nip.
Japanese Patent Application Laid-open No. 2001-318555 discloses a pressure release mechanism in the form of a cam attached to a release lever, which acts on the direct pressure exerting lever to release the pressure exerted by the fixing members. Japanese Patent Application Laid-open No. 2000-214718 discloses a mechanism where the opening and closing of the cover releases the pressure exerted by the fixing members.
The automatic jam releasing cam disclosed in Japanese Patent Application Laid-open No. H5-173446 requires a dedicated motor to drive the cam, making the apparatus heavier as well as costlier.
In response to the demand for a more compact and high speed image forming apparatus, the diameter of the fixing rollers and belts have been reduced, resulting in a narrower nip portion and speedier transit of the sheet through the nip potion. Consequently, the amount of heat applied to the recording sheet by the fixing rollers tends to be inadequate. Therefore, a stronger pressure than in the conventional apparatus must be applied to ensure that the nip portion is effective. As a result, the user must use more power for operating the conventional manual pressure releasing lever which makes the operation difficult.
In the configuration disclosed in Japanese Patent Application Laid-open No. 2001-318555, if stronger pressure is applied, the frictional force between the cam and the direct pressure exerting lever increases, decreasing the slidability and increasing the friction between the sliding contact surfaces.
In the mechanism disclosed in Japanese Patent Application Laid-open No. 2000-214718, if stronger pressure is applied, the frictional force between the release lever and the cover increases, decreasing the slidability and increasing the friction between the sliding contact surfaces.
To provide solutions to the problems described above, a fixing device is disclosed in Japanese Patent Application Laid-open No. 2006-48005. Concretely, a fixing device that clamps a recording sheet between a first fixing member and a second fixing member and fixes a toner image on the recording sheet by applying heat and pressure, includes a pressure exerting lever that causes the first fixing member to exert pressure on the second fixing member, the pressure exerting lever having a first end and a second end, the first end being rotatably supported by a first pivot; a pressure releasing lever that is rotatably supported against the pressure exerting lever; a locking member having a first end and a second end, the first end being rotatably supported against the first end of the pressure releasing lever; and an elastic member that is hooked to the second end of the locking member and that pulls the locking member in a locking direction. By turning the pressure releasing lever, a position of the locking member is switched between a pressure exertion position where the first fixing member exerts pressure against the second fixing member via the pressure exerting lever, and a pressure release position where the fixing member separates from the second fixing member. In the pressure exertion position, the elastic member pulls the locking member and the second end of the pressure exerting lever in the locking direction.
The fixing device includes a pair of fixing members, a pressure exerting lever that exerts pressure on one fixing member, pressing it towards the other, a pressure releasing lever rotatably supported against the pressure exerting lever, a locking member whose one end is rotatably fitted to the pressure releasing lever, and hooked to the other end of the locking member an elastic member that pulls the locking member in a locking direction. When the pressure releasing lever rotates, the locking member shifts between a pressure contact position where one fixing member exerts pressure on the other fixing member via the pressure exerting lever, and a pressure releasing position where there is a gap between the two fixing members. Thus, the need for a cam or a cam driving unit is obviated, and a light and inexpensive apparatus can be realized. When the pressure releasing lever rotates, the locking member, whose one end is engaged with the pressure releasing lever, also shifts its position. Therefore, the elastic member extends minimally and the user needs to exert very little operating force. The pressure exerting lever and the locking member are locked at a position far from rotational center of the pressure exerting lever and the elastic member is configured to pull the locking member. Therefore, the component sliding over the locking member receives decreased frictional force and has high durability.
However, the core end of the fixing member is rotatably supported by a shaft bearing. The pressure exerting lever presses against the shaft bearing, causing one fixing member to come in pressure contact with the other fixing member. Generally, a ball bearing (or slide bearing) is used as shaft bearing. In either case, the shaft bearing deforms and is damaged because of a localized force exerted on the outer periphery of the shaft bearing, resulting in the fixing member not rotating smoothly, and therefore causing problems in locking. Particularly, in a mechanism provided keeping in mind the convenience of the user whereby the fixing members assume pressure contact and pressure release positions with the opening and closing of the cover, the shaft bearing receives severe impact via the pressure exerting lever during pressure exertion when the cover is tightly shut, making the shaft bearing even more vulnerable to deformation. Further, apart from jam release, the cover can be opened and closed for a number of other reasons. Thus, repeated pressure exertion causes the shaft bearing to receive impact that many times, further increasing the risk of deformation.
Even if the fixing device is configured such that the pressure exerting lever and the shaft bearing are in contact, due to the sheer number of components in the fixing device, a slight distortion can occur in the stacking of the components, resulting in a point contact between the pressure exerting lever and the shaft bearing and a localized force is exerted. Due to the variation in the elastic member used in the fixing device, force exceeding the load capacity of the shaft bearing is exerted on the shaft bearing when opening and closing the cover, damaging the shaft bearing.
Further, because the core end of the fixing member is rotatably supported by the shaft bearing, and the pressure exerting lever pushes the shaft bearing to cause one fixing member to exert pressure on the other fixing member, the shaft bearing does not necessarily move with the pressure exerting lever, particularly during pressure release. Thus, even if the pressure exerting lever is at the pressure releasing position, one fixing member may well be touching the other fixing member. In an arrangement of the fixing members where the upper fixing member is a fixing roller and the lower fixing member is a pressure exerting roller, when the pressure exerting lever shifts to the pressure releasing position, the pressure exerting roller shifts downwards away from the fixing roller due to self weight. However, in an arrangement where the fixing roller and the pressure exerting roller are disposed side by side, the two fixing members are touching each other even if the pressure exerting lever shifts to the pressure releasing position.
When the image forming apparatus stops due to the jamming of the transfer sheet, there are instances when the transfer sheet is stuck in the fixing device. When such a transfer sheet, bearing thereon unfixed toner image downstream of the fixing nip, is pulled from the side upstream of the fixing nip with the fixing members touching each other (that is, with no gap between them), the unfixed toner from the transfer sheet is smeared on the fixing members. As a result, faulty image is formed when the next transfer sheet passes through the fixing members. Further, it is not easy to pull out the transfer sheet intact from the nip.